Method for dissolving a silicon dioxide layer

ABSTRACT

This disclosure relates to a method for dissolving a silicon dioxide layer in a structure, including, from the back surface thereof to the front surface thereof, a supporting substrate, the silicon dioxide layer and a semiconductor layer, the dissolution method being implemented in a furnace in which structures are supported on a support, the dissolution method resulting in the diffusion of oxygen atoms included in the silicon dioxide layer through the semiconductor layer and generating volatile products, and the furnace including traps suitable for reacting with the volatile products, so as to reduce the concentration gradient of the volatile products parallel to the front surface of at least one structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. §371 ofInternational Patent Application PCT/IB2014/000250, filed Mar. 3, 2014,designating the United States of America and published as InternationalPatent Publication WO 2014/155166 A1 on Oct. 2, 2014, which claims thebenefit under Article 8 of the Patent Cooperation Treaty and under 35U.S.C. §119(e) to French Patent Application Serial No. 1300706, filedMar. 25, 2013, the disclosure of each of which is hereby incorporatedherein in its entirety by this reference.

TECHNICAL FIELD

This disclosure relates to a method for dissolving a silicon dioxidelayer in a structure of the semiconductor-on-insulator type.

BACKGROUND

A dissolution method known from the prior art, and illustrated in FIGS.1A and 1B, is a method for dissolving a silicon dioxide layer in astructure 5 of the semiconductor-on-insulator type, comprising, from itsrear face 6 to its front face 7, a supporting substrate 8, the silicondioxide layer 9 and a semiconductor layer 10. The front face 7corresponds to the free surface of the semiconductor layer 10.

A person skilled in the art will find a technical description of such amethod in the articles by Kononchuk (Kononchuk et al., “Novel trends inSOI technology for CMOS applications,” Solid State Phenomena, Vols.156-158 (1010) pp. 69-76, and Kononchuck et al., “Internal Dissolutionof Buried Oxide in SOI Wafers,” Solid State Phenomena, Vols. 131-133(2008) pp. 113-118).

This dissolution method may be implemented in a furnace 1, illustratedin FIG. 2, in which a plurality of structures 5 are held on a support 4so that the support 4 is suitable for holding the structures 5 with apredetermined distance, typically a few millimeters, between eachstructure 5, the front face 7 of a structure 5 being opposite the rearface 6 of the structure 5 adjacent to the front face 7.

The structures 5 are subjected to a non-oxidizing atmosphere. Thenon-oxidizing atmosphere is provided by a continuous flow of inert orreducing gas. The flow of inert gas enters the furnace 1 through aninlet 2, and emerges therefrom through an outlet 3.

The use of such a heat treatment causes the diffusion of oxygen atomsincluded in the silicon dioxide layer 9 through the semiconductor layer10. The reaction of the oxygen atoms with the semiconductor layer 10generates volatile products comprising semiconductor monoxide (ScO).However, the gases present on the surface of the structures 5, inparticular, the volatile products generated, have an influence on thedissolution.

Thus, the semiconductor monoxide slows the dissolution reaction when itsconcentration on the surface of the structures 5 increases.

The composition of the atmosphere of the furnace 1 is not homogeneous.This is because of the small spacing between the structures 5. Thevolatile products are discharged solely by diffusion at the edge of thestructures 5. The result of this is an accumulation of the volatileproducts that is greater at the center of the surfaces of the structure5 than at their edge. This means that the dissolution reaction is morerapid at the periphery than at the center of the structures 5.

Moreover, the atmosphere of the furnace 1 is obtained by a constant flowof an inert or reducing gas. The flow of gas entrains, from its entry atinlet 2 into the furnace 1 to its discharge at outlet 3, at least someof the volatile products. Consequently, during its path through thefurnace 1, the flow of gas becomes loaded with volatile products.

Depending on their location in the furnace, the structures are,therefore, subjected to a variable concentration of volatile products.

Finally, the flow of gas may contain small quantities of oxygen.

Since a complete absence of oxygen in the gas flow would require usingvery complex means, a small percentage of oxygen in the gas flowentering the furnace is tolerated.

The oxygen included in the atmosphere of the furnace 1 limits thedissolution of the layer of silicon dioxide 9 and degrades the roughnessof the free surface of the semiconductor layer 10.

The oxygen contained in the gas flow reacts preferentially with thestructures 5 close to the gas inlet 2. The gas flow is, therefore,depleted of oxygen from the inlet 2 of the furnace 1 toward the outlet3.

This non-homogeneity of the atmosphere of the furnace 1 results insignificant variabilities on the characteristics of the structures 5.

The main drawback of this dissolution method is that the non-homogeneityof the atmosphere of the furnace 1 leads to a degradation in theuniformity of thickness of the silicon dioxide layer 9 and of thesemiconductor layer 10, as illustrated in FIG. 1A. This is because, atthe end of the heat treatment, the thickness of the silicon dioxidelayer 9 and the thickness of the semiconductor layer 10 are greater atthe center than at the edge of the structure.

Another drawback of this dissolution method is that it is not uniformfor all the structures 5 of the semiconductor-on-insulator typecontained in the furnace 1. This is because the silicon dioxide layer 9is not dissolved in the same proportions from one structure 5 toanother.

The aforementioned drawbacks are not observed in a furnace 1 containinga single structure 5. However, given the relatively long heat treatmenttimes and for economic reasons, executing such a method in a furnace 1containing only one structure 5 cannot be envisaged from an industrialpoint of view.

However, some applications require having recourse to a silicon dioxidelayer 9 with a thickness of less than 50 nm so as to be able to apply,for example, an electrical voltage exerted on devices produced in or onthe semiconductor layer 10. A very precise control of the thickness ofthe silicon dioxide layer is then necessary.

Moreover, the structures 5 designated by the term “FDSOI,” standing for“fully depleted silicon-on-insulator,” are particularly advantageous forproducing electronic components such as FDMOS (“fully depleted metaloxide semiconductor”) transistors, the channel of which is formed in oron the semiconductor layer 10.

Because of the extreme fineness of the thickness of the semiconductorlayer 10 (i.e., around 10 nm), the threshold voltage of the transistor(usually denoted Vt), which depends on this thickness, is very sensitiveto the variations in thickness of the semiconductor layer 10.

One aim of the disclosure is, therefore, to propose a method fordissolving a silicon dioxide layer affording precise control of thethicknesses of the semiconductor and silicon dioxide layers.

BRIEF SUMMARY

This disclosure aims to completely or partially remedy theaforementioned drawbacks, and relates to a method for dissolving asilicon dioxide layer in a structure of the semiconductor-on-insulatortype, comprising, from its rear face to its front face, a supportingsubstrate, the silicon dioxide layer and a semiconductor layer, thedissolution method being implemented in a furnace in which a pluralityof structures are held on a support, the support being suitable formaintaining a predetermined distance between each structure, the frontface of a structure being opposite the rear face of the structureadjacent to the front face, the atmosphere of the furnace being anon-oxidizing atmosphere, the dissolution method causing the diffusionof oxygen atoms included in the silicon dioxide layer through thesemiconductor layer and generating volatile products resulting from thereaction of the oxygen atoms with the semiconductor layer, the methodbeing remarkable in that the furnace comprises traps suitable forreacting with the volatile products so as to reduce the concentrationgradient of the volatile products parallel to the front face of at leastone structure.

In the present text, the vertical direction is defined as beingperpendicular to the ground on which the furnace is installed. The terms“upper” and “lower” are defined with respect to this vertical direction.

Concentration gradient of the volatile products parallel to the frontface of a structure means the variation in the concentration of thevolatile products in the space lying between the front face of thestructure and the rear face of the structure adjacent to the front face,and in directions lying in a plane parallel to the front face of thestructure.

In contradistinction, a concentration gradient of the volatile productsperpendicular to the front face of the structure is defined as being thenon-uniformity of the concentration of volatile products in the spacelying between the front face of the structure and the rear face of thestructure adjacent to the front face, and in a direction perpendicularto the front face of the structure. Thus, the arrangement in the furnaceof traps suitable for reacting with the volatile products makes itpossible to absorb the products. The result is a reduction in theconcentration gradient of the volatile products parallel to the frontface of each structure.

Consequently the dissolution kinetics of the silicon dioxide layer issubstantially equal at every point on the structure. Moreover, theabsorption of the volatile products makes it possible to havesubstantially the same concentration of volatile products in thevicinity of the front face of each structure. Thus, the dissolutionmethod is substantially uniform from one structure to another.

According to one embodiment, the traps are disposed on the rear face ofthe structures of the semiconductor-on-insulator type. Thus, the trapsare as close as possible to the front face of each structure. Suchproximity of the traps and the front surface of the structures affordsbetter efficacy of the trapping of the volatile products. Moreover, thefront face of each structure is exposed uniformly to the layercomprising the traps.

The concentration of volatile products on the surface of each structureis, thus, more uniform and, therefore, the parallel gradient is reduced.Furthermore, the arrangement of the traps in the immediate vicinity ofthe front face of each structure limits the quantity of volatileproducts entrained by the gas flow.

Consequently, the kinetics of the dissolution method is substantiallyequivalent for each of the structures. Thus, since the dissolutionreaction is no longer limited by the distance separating the successivestructures, it is possible to increase the loading capacity of thefurnace in order to execute the dissolution method. Moreover, thisembodiment does not require any modification to the furnace.

According to one embodiment, the traps disposed on the rear face areincluded in a layer with a thickness greater than 30 nm, preferablygreater than 50 nm.

According to another embodiment, the traps are included in a coatingcompletely or partially covering the support. Thus, it is not necessaryto add manufacturing steps to the structures for executing thedissolution method. Moreover, since the support is brought out of thefurnace at each dissolution process, it can easily be coated with asuitable trapping material.

According to one embodiment, the traps are included in trapping layersat least partially covering the front face of substrates referred to as“trap substrates.” The trap substrates are disposed on the support, eachtrap substrate being positioned in place of a structure of asemiconductor type, and inserted between two structures of thesemiconductor-on-insulator type, the front face of the trap substratebeing opposite the rear face of the structure of thesemiconductor-on-insulator type. Thus, it is not necessary to addmanufacturing steps to the structures in order to execute thedissolution method.

Moreover, with the trap substrates being brought out of the furnaceafter each dissolution process, it is possible to form new traps on thesupport when the latter are saturated with volatile products.

According to one embodiment, the traps are included in a coating whollyor partly covering the internal wall of the furnace. “Inside of thefurnace” means the space in which the support holding the structures isintroduced during the execution of the dissolution method.

According to one embodiment, the reaction between the volatile productsand the traps is a reaction of absorption of the volatile surfaces bythe traps. Thus, the absorption of the volatile products by the trapsprevents contamination of the furnace, support or structures.

According to one embodiment, the traps comprise silicon dioxide. A thinlayer comprising silicon dioxide disposed on the rear face of thestructure, and with a thickness greater than 30 nm, or even greater than50 nm, will withstand a dissolution process at a temperature of between900° C. and 1300° C. On the other hand, the native oxide naturallypresent on the rear face of a silicon-on-insulator structure, throughits small thickness and its chemical composition, will evaporate duringsuch treatment and cannot fulfill the role of a trap. In this regard, aperson skilled in the art will find a technical description of theevaporation of the oxide in a non-oxidizing atmosphere in E. Bussmann etal., “Thermal instability of silicon-on-insulator thin films measured bylow-energy electron microscopy,” Innovation in Thin Film Processing andCharacterization, vol. 12, 012016, 2010.

Furthermore, silicon dioxide is compatible with the methods formanufacturing semiconductor structures.

According to one embodiment, the traps comprise at least one of thefollowing materials: tungsten, aluminium nitride, and alumina. Thus,these materials have the advantage of being very stable undertemperature.

According to one embodiment, the semiconductor layer comprises silicon.

According to another embodiment, the semiconductor layer has a thicknessgreater than 100 nm, preferentially greater than 200 nm, even morepreferentially greater than 300 nm. Thus, such thicknesses of thesemiconductor layer slow down the dissolution reaction. Consequently,the traps have time to react effectively with the volatile products.

According to one embodiment, the layer of silicon dioxide has athickness of less than 50 nm, preferably less than 25 nm, even morepreferentially less than 15 nm.

According to another embodiment, the atmosphere of the furnace comprisesat least one species chosen from argon and dihydrogen.

According to one embodiment, the temperature of the furnace ismaintained at between 900° C. and 1300° C.

According to another embodiment, traps suitable for reacting with thedioxygen included in the atmosphere of the furnace are disposed in thefurnace.

According to one embodiment, the traps intended to react with dioxygenare silicon substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages will emerge from the following descriptionof embodiments of a dissolution method according to this disclosure,given by way of non-limitative examples with reference to theaccompanying drawings, in which:

FIGS. 1A and 1B are schematic representations of a structure treated bya method for dissolution of a structure of thesemiconductor-on-insulator type according to the techniques known fromthe prior art;

FIG. 2 is a schematic representation of a furnace intended for executinga heat treatment for dissolution of a silicon dioxide layer according tothe techniques known from the prior art;

FIGS. 3A and 3B are schematic representations of a structure treatedaccording to the disclosure;

FIG. 4 is a schematic representation of a furnace intended for executinga heat treatment for dissolution of a silicon dioxide layer according tothe disclosure;

FIG. 5 is a schematic representation of a structure treated according toone embodiment of the disclosure; and

FIG. 6 is a schematic representation of a trap and substrate accordingto the disclosure.

DETAILED DESCRIPTION

For the various embodiments and for reasons of simplification of thedescription, the same references will be used for identical elements orones fulfilling the same function.

The dissolution method illustrated in FIGS. 3A and 3B is a method fordissolving a silicon dioxide layer 90 in a structure 50 of thesemiconductor-on-insulator type.

The structure 50 of the semiconductor-on-insulator type comprises, fromits rear face 60 to its front face 70, a supporting substrate 80, thesilicon dioxide layer 90 and a semiconductor layer 100.

The dissolution method is implemented in a furnace 11, illustrated inFIG. 4, in which a plurality of structures 50 are held on a support 40,the structures being parallel to one another. The front face 70 of astructure 50 is opposite the rear face 60 of the structure 50 adjacentto the front face 70. The support 40 is suitable for holding thestructures 50 with a predetermined distance between each structure 50.The predetermined distance between each structure 50 may be less than 15nm, preferably less than 10 nm. The atmosphere of the furnace 11 isanon-oxidizing atmosphere.

The dissolution method causes the diffusion of oxygen atoms included inthe silicon dioxide layer 90 through the semiconductor layer 100.Volatile products resulting from the reaction of the oxygen atoms withthe semiconductor layer 100 are generated. The volatile productscomprise semiconductor monoxide (ScO).

Traps 110, suitable for reacting with the volatile products, aredisposed in the furnace 11 so as to reduce the concentration gradient ofthe volatile products parallel to the front face 70 of at least onestructure 50. The oxygen content of the non-oxidizing atmosphere ispreferably less than 10 ppm (ppm: parts per million). The non-oxidizingatmosphere of the furnace 11 is provided by an inert or reducing gasflow. The inert or reducing gas flow can enter the furnace 11 through aninlet 20, and emerges therefrom through an outlet 30.

The atmosphere of the furnace 11 may comprise at least one specieschosen from one of the following species: argon and dihydrogen.

During the dissolution process, the temperature of the furnace 11 can bemaintained at a temperature between 900° and 1300° C., for example 1150°C.

The conditions of implementation of this dissolution method, inparticular, its duration, can be adapted in order to partially dissolvethe silicon dioxide layer 90.

The supporting substrate 80 may comprise at least one of the followingmaterials: silicon, germanium, alumina, and quartz.

The semiconductor layer 100 may comprise at least one of the followingmaterials: silicon, germanium, and silicon germanium alloy.

According to a particularly advantageous embodiment, and as illustratedin FIG. 5, the traps 110 are disposed on the rear face 60 of thestructures 50. Advantageously, a coating comprising the traps 110 may beformed on the rear face 60 of the structures 50. Thus, the traps 110 areas close as possible to the front face 70 of each structure 50. Suchproximity of the traps 110 and the front face 70 of the structures 50affords better efficacy of the trapping of the volatile products.Moreover, the front face 70 of each structure is exposed uniformly tothe layer comprising the traps 110. The concentration of the volatileproducts on the front face 70 of each structure 50 is, thus, moreuniform and, therefore, the parallel gradient is reduced. Furthermore,the arrangement of the traps 110 in the immediate vicinity of the frontface 70 of each structure 50 limits the quantity of volatile productsentrained by the gas flow. Consequently, the variations in concentrationof volatile products in the furnace 11 are reduced. Thus, thevariabilities in dissolution from one structure 50 to another arereduced. Consequently, the silicon dioxide layer 90 is dissolvedsubstantially in the same proportions from one structure 50 to another.

Alternatively, or in a complementary manner, and as illustrated in FIG.6, the traps 110 may be included in trapping layers on the front face 70of the substrates referred to as trap substrates 120. The trapsubstrates 120 are disposed on the support 40 in place of certainstructures 50. Consequently, the variations in concentration of volatileproducts in the furnace 11 are reduced. Thus, the variabilities ofdissolution from one structure 50 to another are reduced.

Particularly advantageously, the traps 110 disposed on the rear face 60of the structures 50 or on the front face of the trapping layers of thetrap substrates 120 may comprise silicon dioxide. On the other hand, thenative oxide naturally present on the rear face of a structure of thesilicon-on-insulator, through its small thickness and its chemicalcomposition, evaporates during such treatment and cannot fulfill therole of a trap.

The traps 110 comprising silicon dioxide are advantageously formed bythin film deposition techniques. Among film deposition techniques,low-pressure vapor deposition and plasma-activated vapor depositiontechniques known to persons skilled in the art will be cited.

Alternatively, the formation of a silicon dioxide film on the rear face60 of a supporting substrate 80 made from silicon or on the front faceof a trap substrate 120 can be advantageously executed by thermaloxidation.

The film formed on the rear face 60 of a structure 50 or on the frontface of a trap substrate 120 may have a thickness greater than 30 nm,preferentially greater than 50 nm.

Alternatively, the traps 110 may comprise at least one of the followingmaterials: titanium, tungsten, aluminium nitride, and alumina.

These materials can absorb the semiconductor monoxide formed during theprocess of dissolution of the silicon dioxide layer 90.

These materials may be formed in the form of films by film depositiontechniques known to persons skilled in the art. For example, evaporationtechniques are particularly well suited to the formation of films oftitanium and tungsten. Aluminium nitride and alumina are advantageouslyformed by chemical vapor deposition or atomic layer depositiontechniques.

Alternatively, or in a complementary fashion, traps 110 may be includedin a coating covering all or part of the support 40. Thus, the traps 110are close to the front face 70 of the structures 50. Consequently, thevolatile products are effectively trapped. Particularly advantageously,the traps 110 comprise silicon dioxide. The traps 110 comprising silicondioxide are advantageously formed by vapor deposition techniques or bythermal oxidation. The coating formed on the support 40 may have athickness greater than 50 nm, or even greater than 500 nm.

Alternatively, the traps 110 may comprise at least one of the followingmaterials: titanium, tungsten, aluminium nitride, and alumina.

These materials may be formed in the form of a film by film depositiontechniques known to persons skilled in the art; for example, chemicalvapor deposition or atomic layer deposition techniques.

In a complementary fashion, the traps 110 are included in a coatingwholly or partly covering the internal wall of the furnace 11.Particularly advantageously, the traps 110 comprise silicon dioxide.

Alternatively, the traps 110 may comprise at least one of the followingmaterials: titanium, tungsten, aluminium nitride, and alumina.

Particularly advantageously, the semiconductor layer 100 has a thicknessgreater than 100 nm, preferentially greater than 200 nm, even morepreferentially greater than 300 nm. For such thicknesses ofsemiconductor layer 100, the dissolution speed is less than 0.5Å/minute.

Thus, the volatile products formed during the dissolution process havethe time to diffuse toward the traps. Consequently, the variations inconcentration of volatile products are reduced. Advantageously, thelayer of silicon dioxide 90 has a thickness of less than 50 nm,preferentially less than 25 nm, even more preferentially less than 15nm.

Advantageously, trap substrates 120 suitable for reacting with thedioxygen included in the atmosphere of the furnace 11 may also bedisposed in the furnace 11. The trap substrates 120 intended to reactwith dioxygen may be silicon substrates disposed on the substrate 40 inplace of certain structures 50. Preferably, the silicon substratesintended to react with the dioxygen included in the atmosphere of thefurnace are disposed close to the inert or reducing gas inlet.

Even more preferentially, the silicon substrates are disposed upstreamof the inert or reducing gas flow with respect to the structures 50.Thus, the oxygen included in the inert or reducing gas flow reacts withthe traps 120 before reaching a structure 50.

The method for dissolving a layer of silicon dioxide 90 according to thedisclosure makes it possible to make the composition of the atmosphereof the furnace 11 homogeneous. The accumulation of volatile products isreduced.

Consequently, it is possible to limit the degradation in the uniformityand thickness of the silicon dioxide layer 90 and of the semiconductorlayer 90 found in the prior art. Moreover, the method according to thedisclosure allows a more uniform dissolution from one structure 50 toanother, compared with the method of the prior art.

1.-15. (canceled)
 16. A method for dissolving a silicon dioxide layer ina semiconductor-on-insulator type structure, the method comprising:providing semiconductor-on-insulator type structures on a support in afurnace, each of the semiconductor-on-insulator type structures having arear face and a front face and including a supporting substrate, asilicon dioxide layer over the supporting substrate, and a semiconductorlayer on a side of the silicon dioxide layer opposite the supportingsubstrate, the support being configured for holding thesemiconductor-on-insulator type structures with a predetermined distancebetween the semiconductor-on-insulator type structures, the front faceof one of the semiconductor-on-insulator type structures being oppositethe rear face of another of the semiconductor-on-insulator typestructures adjacent to the front face of the one of thesemiconductor-on-insulator type structures; providing a non-oxidizingatmosphere within the furnace; heating the semiconductor-on-insulatortype structures within the furnace and causing the diffusion of oxygenatoms included in the silicon dioxide layers through the semiconductorlayers and generating volatile products resulting from the reaction ofthe oxygen atoms with the semiconductor layers; and reacting thevolatile products with traps located within the reactor and reducing aconcentration gradient of the volatile products in a direction parallelto the front face of at least one of the semiconductor-on-insulator typestructures.
 17. The method of claim 16, wherein the traps are disposedon the rear faces of the semiconductor-on-insulator type structures. 18.The method of claim 16, wherein the traps are included in a coatingcompletely or partly covering the support.
 19. The method of claim 16,wherein the traps are included in trapping layers on front faces of trapsubstrates, the trap substrates disposed on the support, each trapsubstrate being inserted between two of the semiconductor-on-insulatortype structures, the front face of each trap substrate being oppositethe rear face of one of the semiconductor-on-insulator type structure,respectively.
 20. The method of claim 16, wherein the traps are includedin a coating completely or partly covering an internal wall of thefurnace.
 21. The method of claim 20, wherein reacting the volatileproducts with the traps located within the reactor comprises absorptionof the volatile products by the traps.
 22. The method of claim 21,wherein the traps comprise silicon dioxide.
 23. The method of claim 21,wherein the traps comprise at least one material of the group consistingof titanium, tungsten, aluminum nitride, and alumina.
 24. The method ofclaim 16, wherein the semiconductor layers of thesemiconductor-on-insulator type structures comprise silicon.
 25. Themethod of claim 16, wherein the semiconductor layers of thesemiconductor-on-insulator type structures have a thickness greater than100 nm.
 26. The method of claim 25, wherein the semiconductor layers ofthe semiconductor-on-insulator type structures have a thickness greaterthan 200 nm.
 27. The method of claim 26, wherein the semiconductorlayers of the semiconductor-on-insulator type structures have athickness greater than 300 nm.
 28. The method of claim 16, wherein thesilicon dioxide layers of the semiconductor-on-insulator type structureshave a thickness of less than 50 nm.
 29. The method of claim 16, whereinthe silicon dioxide layers of the semiconductor-on-insulator typestructures have a thickness of less than 25 nm.
 30. The method of claim16, wherein the silicon dioxide layers of the semiconductor-on-insulatortype structures have a thickness of less than 15 nm
 31. The method ofclaim 16, wherein providing a non-oxidizing atmosphere within thefurnace comprises providing at least one of argon and dihydrogen withinthe furnace.
 32. The method of claim 16, wherein heating thesemiconductor-on-insulator type structures within the furnace comprisesmaintaining a temperature of the furnace at a temperature between 900°C. and 1300° C.
 33. The method of claim 16, wherein reacting thevolatile products with traps located within the reactor comprisesreacting dioxygen with trap substrates disposed in the furnace.
 34. Themethod of claim 33, wherein the trap substrates are silicon substratesdisposed on the support.